Subjective images and impressions of light, color or pattern generated within the eyes and brain rather than by optical images from outside are commonly termed "phosphenes" and can be formed in various ways. For example, strobe light or electrical pulses applied to the scalp or eyes, or fluctuating magnetic fields in the vicinity of the brain can create such impressions. Furthermore, pressure on the eye, a blow to the brain or direct electrical impulses may cause similar experiences. Generation of such phosphenes may range from simple one-color flashes to multicolored, complex visual patterns, such as, snow flakes, diamonds, whorls, checks or stripes of various colors or hues in addition to many forms and colors that the imagination might create. In general the patterns would not appear to have any direct relation to the shape or the pattern of the light source or force creating them in the mind and therefore do not appear to be representative of the source which evokes such patterns. It has been theorized that the patterns of phosphenes may be related to the geometry of the eye or brain; and when intentionally evoked may take on as many forms or colors as the imagination might create particularly by constant practice.
Devices which have been developed for producing phosphenes have suffered from a number of defects: Principally, known devices have not taken into consideration the most desirable or optimum conditions for maximizing phosphene generation or reaction while at the same time minimizing flicker sickness. In order to optimize phosphene generation using a strobe light, the maximum average intensity of light must be achieved without reaching pear brightness levels which could be damaging to the eye and the frequency of the pulses must be below the flicker fusion frequency. Generally, the apparent brightness of light falling on the eyes is an average of the light passed over a sampling period roughly equal to the flicker fusion frequency (i.e.: the minimum frequency produced by a flashing light which is necessary to insure continuous visual sensation). As a result, phosphene generators which employ light pulses of relatively short duration in relation to the duration of darkness would require extremely or intensely bright light pulses and at a level of brightness which could be damaging to the eye. The German patent to Sownberger No. 866,905 granted in 1953 discloses a stereoscopic device in which light from an external source has been converted into light pulses of relatively short duration by alternately blocking the passage of light to the eye by placing a rotatable slotted member in front of the eyes and rotating about an axis perpendicular to the passage of light. Other rotating slot devices representative of the prior art are shown in United States Letters Patent to Jarrow U.S. Pat. No. 2,387,758, Denecke U.S. Pat. No. 3,049,962 and Neasham U.S. Pat. No. 3,415,598 as well as British Patent to Bankl No. 012,797 and the French Patent to Georges et al No. 1,297,887. Further, various types of stereoscopic viewers containing light modulating features which operate with motor drives and switching frequencies above the flicker fusion frequency and, for instance, are shown in U. S. Letters Patent to Dockhorn U.S. Pat. Nos, 2,810,318; 3,825,263, and 2,825,264 as well as Kratomi U.S. Pat. No. 3,737,567. None of the foregoing discloses or contemplates an arrangement wherein maximum average light intensity is achieved while minimizing flicker sickness in the manner devised according to the present invention and specifically through a unique manner and means by which the size and speed of a rotating, oscillating or reciprocating element in front of the eyes are so correlated and coordinated as to produce optimum, safe conditions for phosphene generation in a simplified dependable manner.